In the automatic transmission for motor vehicles known from the prior art, shifting elements and clutches or brakes for adjusting a certain reduction ratio of the automatic transmission are controlled by a hydraulic system with preset time-pressure profiles. The different components of the transmission required for this are coordinated via an electronic unit connected with the hydraulic system which, in accordance with the currently existing operating situation of the motor vehicle and eventual directives of the driver""s wish, adequately controls a gear shift to obtain a desired shifting quality.
In a customary gear shift, especially with an engaging shifting element or an engaging clutch, a time-pressure profile can be divided in a filling phase and a shifting phase, it being, in turn, possible to subdivide the filling phase in a rapid filling phase and a filling equalization phase. The shifting phase, which follows the filling phase of the shifting element, constitutes the range of a time-pressure profile of an engaging shifting element in which the shifting element is loaded, via a pressure ramp, with a differential pressure until reaching a pressure outside a gear shift which leads to transmission of a required or desired torque of the shifting element.
In order to satisfy the constantly increasing requirements on the functionality of the automatic transmission, the same as the efforts to increase the comfort in shifting, it has for that purpose been changed over to increasing the number of gears to be shifted and to provide a more compact design of the transmissions. Especially the high number of gear steps of the transmissions results in a shifting element or a clutch is often used to shift different gears.
To improve the variability and spontaneity of the automatic transmissions, it has, in addition, been generally provided that a driver can manually engage in the shifting behavior and thus in preset shifting sequences of an automatic transmission. Besides, in the electric control unit shifting strategies are usually deposited which allow the discontinuation or cancellation of gear shifts in certain operating and driving situations.
The steps result, however, in that an engagement and disengagement of the clutches at the right moment, that is, without impairing the driving comfort or the shifting quality, become increasingly difficult. It is, in particular, problematic, for example, when a clutch with too low filling is shifting in a power flow of a driving train of the motor vehicle. At such an operating point, the clutch is still not capable of transmitting the required or desired torque. After its engagement, if the clutch attains its full transmission capacity, this can be detected by a so-called shifting jerk which must be prevented, on one hand, because of a desired shifting quality and on the other, because of a considerable overload of the clutch. On the contrary, if a clutch reaches its full capacity before the actually provided moment of its engagement, this leads to an interruption of the acceleration which, likewise, represents an impairment of the driving comfort.
To prevent these disadvantages, there have been provided in the practice prohibition times between two consecutive gear shifts before the termination of which times no gear shift is carried out. Thereby is to be ensured that the shifting elements or clutches reach a well-defined state, namely, filled or drained at which the gear shift can be built up free of problems and without receiving the described phenomena that reduce the shifting quality. Besides, to circumvent the above mentioned problems, a discontinuation or a cancellation of gear shifts, that is, the removal of the control pressure on the shifting element in the shifting preparatory or shifting phase, often is not allowed or is allowed only very limitedly so that the shifting operations already introduced have to be carried out first before the start of another gear shift.
It is disadvantageous here that a desired spontaneity of the automatic transmission or of the motor vehicle often cannot be obtained in the driving operation and, in particular, in the case of a driver""s desired directions such as xe2x80x9ckickdownxe2x80x9d following a discontinued overtaking operation related to an introduced downshift operation, the desired or required output torque is not immediately available.
The problem on which this invention is based is to make available a method for operating an automatic transmission with which a high spontaneity can be achieved without damages to a desired shifting quality.
With the inventive method, via an algorithm stored in the electronic control unit, it is now possible to determine an actual operation point of the shifting element concerned as function of a time-pressure profile according to which a shifting element has last been controlled and of an associated profile representative of the operative point of the shifting element with regard to a filling ratio and/or a position of a piston of the shifting element.
Thereby is created a kind of filling level indicator or filling pattern which offers the advantage that at every moment in the operation of the automatic transmission the actual state of each shifting element is known. In particular, it is possible in each shifting element to automatically determine during engagement the right filling time and during disengagement the right draining time, wherefrom the possibility results of a precise control adapted to the actual operating point during a shifting operation that follows.
The inventive method especially offers the advantage, in case of a gear shift following a discontinued gear shift in which the same shifting element took part, that the new shifting operation is built up on the actual operating point or operating state of the shifting element and the required control for effecting the second gear shift is carried out spontaneously and without loss of shifting quality with a control pressure suitable to the situation.
The transmission can be very precisely coordinated when a transmission applicator presupposes important physical marginal conditions of the shifting element and control thereof. Thereby is advantageously ensured that the shifting element, for example, at the moment of a desired load take-up of a shifting element, be entirely filled and transmits the required torque. It is conversely ensured that no torque be transmitted any longer at the moment of load delivery when no positive engagement exists.